This invention is related to a heater assembly for a heat engine designed to utilize heat from two distinctly different energy sources and particularly to a heater assembly for a heat engine designed to utilize both solar energy and heat produced by combustion of a fuel such as natural gas.
Heat engines, such as Stirling cycle engines, are capable of converting heat in a working fluid to mechanical output energy. Heat engines are typically coupled to an electrical generator, which converts the mechanical output energy into electricity, or to a device that utilizes the mechanical output energy, such as an irrigation pump or manufacturing equipment. Heater assemblies are the components of heat engines used to transfer heat from an external heat source, such as the sun or a fuel combustor, to an internal working fluid circulating within the heat engine, such as helium or hydrogen. The working fluid undergoes a thermodynamic cycle within the heat engine that converts a portion of the heat energy in the working fluid into mechanical output energy.
One primary application for heat engines is the conversion of solar energy into electricity. In these applications, the heat engine is typically coupled with a solar concentrator and an electrical generator. The solar concentrator is generally a parabolic dish covered with a reflective material, such as glass mirrors, which reflect incident solar radiation and focus this energy toward an energy receptor, which is typically located inside a receiver chamber of a heater assembly attached to the heat engine. The heater assembly typically includes a receiver housing, which forms the receiver chamber and has a receiver aperture or opening which allows the inside of the receiver chamber to be insolated, i.e. exposed to the solar radiation reflected by the solar concentrator. An array of heater tubes attached to the heat engine are located within the receiver chamber and the working fluid circulates through them. The heater tubes absorb the solar radiation and increase the temperature of the working fluid, which is then circulated into the other components of the heat engine where this heat energy is converted into mechanical energy.
A great deal of commercial interest has been expressed in developing large fields of solar powered electrical generators to supply additional quantities of electricity needed during peak electrical demand periods. Peak demand periods for electrical service typically occur during daylight hours during the summer, due in large part to the electrical demands of air conditioning equipment. Because it is during these periods when solar powered electrical generators are typically able to generate maximum quantities of electricity, solar powered electrical generators offer a unique source of electricity for utility companies trying to plan for these fluctuations in electrical service demand.
Utility companies are interested in being environmentally sensitive by using sources of renewable energy, such as solar energy, and reducing the generation of pollutants associated with typical fossil fuel and nuclear power electrical generation systems. They are also interested in reducing the costs associated with constructing, operating and eventually dismantling additional nuclear or fossil fuel powered electricity generating facilities. Fuel costs, particularly the costs of petroleum-based fossil fuels such as fuel oil, have widely fluctuated in the past and utility companies are interested in developing sources of energy that are less subject to these price fluctuations. Fields of solar concentrators combined with Stirling cycle engines connected to electrical generators are currently being evaluated for these types of electricity generation applications.
A significant problem with the vast majority of solar powered electrical generation systems, however, is that they are powered solely by solar energy and solar energy is inherently intermittent. Solar collectors incorporating photovoltaic cell technology, for instance, are unable to utilize any source of energy other than solar energy to produce electricity. Solar energy is inherently intermittent because it is both periodic, because of the diurnal day/night cycle, and random, because of sporadic and often protracted cloudiness that is found virtually everywhere on earth. Even the sunniest areas of the southwest U.S., such as Death Valley, Calif., have an average cloudiness of approximately twenty percent, and this cloudiness can persist for several days in a row. Solar powered electrical generation systems are generally inoperable during these persistently cloudy, overcast periods. Because the total solar energy reaching the ground on a cloudy, overcast day may be one tenth or less of the solar energy reaching the ground on an clear day with no haze or smog, the electricity produced by any type of electricity generating solar collector will depend, in significant part, on the cloudiness of the region the solar collector is installed in. Utility companies and other entities utilizing solar collectors to collect solar energy must therefore constantly factor in a large degree of uncertainty regarding the availability of the power from these systems as well as prepare for the possibility of receiving no power at all from these systems for periods of up to several days in a row.
In addition, extreme weather conditions and maintenance requirements reduce the operating efficiency of typical solar energy collection systems. Thunderstorms, hail, wind blown debris, and high winds can seriously damage solar collection systems. To prevent this type of damage, solar collectors may be inactivated during extreme weather conditions, such as by covering the solar concentrator or rotating the concentrator into a downward facing stowed position. When cleaning or repairing the reflecting surface of a solar concentrator during daylight hours, the concentrator will typically be pointed away from the sun to reduce the possibility of injuring a maintenance worker or damaging the equipment.
To compensate for these fluctuations in power output, many operators of solar powered electrical generation equipment have installed separate fossil-fuel fired backup generation systems or costly, complex and inefficient heat storage systems. The cost and complexity of installing and operating these types of alternative power generation or heat storage systems have significantly reduced the commercial viability of many types of solar powered electrical generation systems.
The inventive heat engine heater assembly has been designed to utilize concentrated solar energy when this energy is available, and to utilize heat produced by combustion of a fuel, such as natural gas, when solar energy is not available. In this way, the hybrid powered electrical generation system is available to produce electricity during cloudy periods, at night, during periods of extreme weather and while maintenance is being performed on the solar concentrator. The inventive heater assembly has been designed to allow the heat engine to be rapidly changed from being solar powered to being combustion powered, and vice versa.
Stirling cycle engines are the primary type of heat engine being evaluated for commercial solar powered electrical generation systems. Stirling cycle engines offer very high thermal efficiency as well as long service free lives. The heat engine used in connection with the inventive heater assembly could include Stirling cycle engine designs and components previously developed by the Assignee of the present invention, Stirling Thermal Motors, Inc., including those described in U.S. Pat. Nos. 4,707,990; 4,715,183; 4,785,633; and 4,911,144, which are hereby incorporated herein by reference.
In contrast to most internal combustion engines, heat engines are typically able to utilize heat from a variety of sources and are not particularly sensitive to the quality of the heat provided. In many cases, the only change required to change heat sources for a heat engine is to install a heater assembly that has been optimized for that particular type of heat source. The internal components of the heat engine may be identical or extremely similar for a wide spectrum of alternative heat sources.
The inventive heater assembly, however, eliminates the need to replace the heater assembly when changing heat sources from solar energy to fuel combustion and vice versa because the heater assembly has been designed to utilize both sources of heat.
The heater assembly incorporates inner and outer heater tube arrays, a receiver housing having a receiver cavity and forming a receiver aperture which allows the receiver cavity to be insolated, a cover for sealing off the receiver aperture, a fuel combustor and a preheater which warms the intake air with heat from the exhaust gases.
The heater tubes arrays are positioned in nested pair of inner and outer arrays that have modified inverted conical frustrum shapes. The individual heater tubes are twisted or swirled in such a manner that identical gaps are maintained between adjacent heater tubes throughout their runs. The inner array of heater tubes connects a number of cylinder extension manifolds to a number of heater tube heads. The outer array of heater tubes connect the heater tube heads to a number of regenerator housing manifolds. Separate heater tubes extend between a cylinder extension manifold and a heater tube head and between a heater tube head and a regenerator housing manifold. Passageways in the heater tube head allow working fluid to flow from the cylinder extension manifold, through the inner array of heater tubes, through the heater tube head, through the outer array of heater tubes and enter the regenerator housing manifold, and vice versa. In the double acting Stirling cycle engine embodiment of the heat engine described herein, the working fluid is constantly shuttled back and forth between the cylinder extension manifolds and the regenerator housing manifolds as the engine operates.
A common design problem associated with the design of a heater assembly for a Stirling cycle engine or other heat engine is how to arrange a plurality of heater tubes emanating from a smaller inner circle of a given radius, r.sub.o, and going to a circle of larger radius, r.sub.f, so that the gap between adjacent tubes is uniform throughout their run. In a direct flame heater head, the uniformity of the gap is important for to obtain desirable external heat transfer characteristics. It is desirable to heat the heater tubes and associated components as evenly as possible to reduce the formation of expansion stresses on the components that can lead to component failures. For a direct-illumination solar receiver, it is advantageous to maintain line contact between adjacent tubes over their entire effective length. This line contact relationship allows a maximum amount of energy to be received for any given set of heater tubes.
When the base circle and the final circle are on the same plane, a constant gap can be obtained by tubes whose centerlines form involutes. Involutes are plane curves formed by the paths of equally spaced points on a line tangent to the root circle as the line is rolled without slip on the circle. A problem arises, however, when there is an axial separation between the base circle and the final circle. When the plane involutes are projected onto an axially symmetric surface between the two circles, say a conical frustrum, the projections are space-curves with gaps between adjacent curves which are, in general, not uniform.
A one-parameter family of axially symmetric surfaces is disclosed herein upon which the space-curves which are projections of plane involutes maintain a uniform gap. These space-curves are particular projections of plane involutes onto a conical frustrum which maintain a uniform gap between adjacent curves.
The inventive heater assembly incorporates a pair of these novel heater tube arrays that are nested closely together to provide an opaque surface to solar radiation. Each member of the outer array of heater tubes is centered within the gap between adjacent members of the inner array of heater tubes when viewed from the central axis of the heater tube arrays. In this way, uniform gaps are also obtained between each member of the outer array and the two closest members of the inner array, thereby further enhancing the heat transfer characteristics of the array when operating in the combustion mode. In addition, the members of the outer array have apparent areas which are the same or larger than the apparent areas of the gaps between adjacent members of the inner array when viewed from the central axis of the heater tube arrays. In this way, the receiver tube arrays present an opaque surface to solar radiation, thereby enhancing the heat transfer characteristics of the array when operating in the solar energy mode and reducing the "dead" area caused when a member of the inner heater tube array shades a member of the outer heater tube array.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.